An insulation resistance test, commonly known as a Megger test, is often used to determine if insulation or connections on a cable system are degrading. For example, a Megger test may be performed to test a power cable that is serially connected to a number of electrical devices, such as lights. One drawback with using such a conventional test is that the test may indicate that there is a problem on the system, but the test is unable to indicate which segment of the cable has a problem. When the cable system spans a long distance, an electrician may take hours to identify the source of the problem through a number of manual interventions and test break points.
An impedance test may also be performed using a Time Domain Reflectometer/Reflectometry (TDR). A TDR test transmits a short rise time pulse along a conductor. If the conductor is of uniform impedance and is properly terminated, the entire transmitted pulse will be absorbed in the far-end termination and no signal will be reflected toward the TDR. Any impedance discontinuities will cause some of the incident signal to be sent back toward the source. The resulting reflected pulse that is measured at the output/input to the TDR is displayed or plotted as a function of time and, because the speed of signal propagation is almost constant for a given transmission medium, can be read as a function of cable length. One of the drawbacks of this test is that in a medium that is not uniform (i.e., many splices exist, transformers are connected in series, etc.), the reflected pulse cannot be used to accurately assess a cable fault.